It would certainly be really difficult. If they did manage to pull it off, it would definitely be restricted to 2D, and would likely either be an overworld maker, or a dungeon maker, as both are quite different tasks. It would be almost impossible though, because Super Mario Maker focuses on minute-long stages; whereas Zelda is a multi-hour-long experience that would take ages to make. Think about it: a Mario level takes probably 2 hours to make properly, but no more than 5 minutes to play through. Scale that up to 2-3 hour dungeons, and the time needed to create them becomes to big to reasonably fit into a game, not to mention the far greater difficulty in designing a good dungeon vs designing a good Mario level.